1. Field of the Invention
The present invention relates to a magnetic energy filter having plural magnetic fields designed to deflect the trajectory of the electron beam from the entrance window to an exit slit.
2. Description of the Related Art
FIG. 4 shows an example of the structure of an electron microscope having electron optics incorporating an OMEGA energy filter. FIG. 5 illustrates the structure of the A-type OMEGA energy filter. FIG. 6 illustrates the structure of the B-type OMEGA energy filter. FIGS. 7(a) and 7(b) illustrate the fundamental trajectory in the A-type OMEGA energy filter. FIGS. 8(a) and 8(b) illustrate the fundamental trajectory in the B-type OMEGA energy filter.
In-column energy filters, such as OMEGA energy filters and Castain-Henry filters, are often used as energy filters connected with electron microscopes in use because the microscope column can be incorporated in the microscope while maintaining the column straight. In an electron microscope having electron optics incorporating the OMEGA energy filter, an electron gun 11 emits an electron beam that is directed to a specimen 14 through condenser lenses 12, as shown in FIG. 4. An observable image of the specimen is projected onto a fluorescent screen 20 via an objective lens 13, an intermediate lens 15, an entrance window 16, an OMEGA energy filter 17, an exit slit 18, and a projector lens 19. In this OMEGA-type energy filter, four magnetic fields M1, M2, M3, and M4, where the beam has radii of curvature R1, R2, R3, and R4, respectively, are arranged to form an xcexa9-shaped trajectory. The electron beam is passed through these magnets in turn such that the outgoing beam is aligned with the incident beam. FIGS. 5 and 6 show two examples of geometry of the magnetic polepieces and electron trajectory. A straight line on which the incident beam and the outgoing beam are aligned with each other or a straight line passing through both entrance window and exit slit is referred to as the xe2x80x9cstraight axisxe2x80x9d herein. The center trajectory of the beam deflected by the magnetic fields of the filter, as shown in FIGS. 5 and 6, is referred to as the xe2x80x9coptical axis indicating the center trajectory of the filterxe2x80x9d herein.
In this way, an instrument having an OMEGA energy filter inserted in or behind the imaging lens system of a transmission electron microscope is used as an apparatus for electron spectroscopic imaging (ESI). In OMEGA energy filters, ALPHA energy filters, and so on, the optical axis of the incident beam is in line with the optical axis of the outgoing beam. Plural magnetic fields are developed. Therefore, such an OMEGA energy filter is inserted in the imaging lens system to deflect the trajectory of the electron beam from the entrance window to the exit slit, and this is called an in-column ESI instrument. On the other hand, a filter in which a single-sector magnet is combined with a multipolar corrector is also available. In this filter, the optical axis of the outgoing beam makes an angle of about 90xc2x0 to the incident beam. Therefore, this filter is mounted behind the microscope column and known as a post-column filter.
An OMEGA energy filter is a typical in-column filter. The prototype of this filter was manufactured by combining a magnetic field prism, an electrostatic mirror, and another magnetic field prism to form an in-column filter (originally known as the Castain-Henry filter) and replacing the electrostatic mirror by a magnetic field prism so that all the deflecting elements were made of magnetic fields. This filter was developed in the 1970s in France and consists of three magnetic fields. Since then, aberration theories of filters have been investigated in Germany. It has been found that use of four magnets is more advantageous than use of three magnets. Subsequent researches have been conducted into systems using four magnets.
In a sector-shaped magnet having a uniform magnetic field, the beam is focused in a direction x parallel to the plane of the magnetic polepieces in which energy dispersion takes place. However, no focusing action occurs in the direction of the magnetic field y. Accordingly, in the case of an OMEGA energy filter, the end surfaces of the magnetic polepieces are tilted to produce a quadruple lens action which focuses the beam in the direction of the magnetic field. The two examples shown in FIGS. 5 and 6 are designed under different optical conditions. The geometry of FIG. 5 is called type A in which three focusing actions take place in the direction x parallel to the plane of the magnetic polepieces and also in the direction of magnetic field y. The geometry of FIG. 6 is called type B in which three focusing actions take place in the direction x parallel to the plane of the magnetic polepieces, and two focusing actions occur in the direction of the magnetic field y. Their differences in fundamental optics can be seen from the trajectory diagrams of the types A and B shown in FIGS. 7 and 8, respectively, where the optical axis indicating the center trajectory of the filter is drawn as a straight line.
In these trajectory diagrams, both trajectories xxcex1 and yxcex2 are trajectories of an electron beam that will finally form a focused electron microscope image on the fluorescent screen. On the other hand, trajectories xxcex3 and yxcex4 are trajectories of an electron beam focused onto the entrance window plane of the filter by the previous stage of lens. After passage through the filter, these trajectories xxcex3 and yxcex4 are focused onto the exit slit plane. On reaching the exit slit plane, the electron beam is sufficiently dispersed according to its energy. The exit slit selects only a desired energy range of the beam. The image on the fluorescent screen is formed by an energy range of the beam passed through the exit slit. If the dispersion is left behind, a blurring will take place. Therefore, the dispersion must disappear on the image plane, which is called the achromatic condition. The OMEGA energy filter has a great feature in that the trajectory is made symmetrical with respect to the center plane, canceling out the aperture aberration on the image plane and distortions.
In the OMEGA energy filter, in order to make some second-order aberrations zero and to reduce the remaining aberrations, the plane between the second magnet M2 and the third magnet M3 is used as a plane of symmetry (center plane). Thus, the beam trajectories before and after the plane of symmetry are rendered symmetrical. In particular, let LL be the distance from the image plane (pupil plane) to the exit slit plane. The image plane (pupil plane) of the incident beam is adjusted to be at a distance of LL from the entrance window plane. Under these conditions, types A and B differ on the trajectory in the y-direction (in the direction of the magnetic field) as follows. With the type A, relations yxcex2=0 and yxcex4xe2x80x2=0 hold on the plane of symmetry as shown in FIGS. 7(a) and 7(b). With the type B, relations yxcex2xe2x80x2=0 and yxcex4=0 hold on the plane of symmetry as shown in FIGS. 8(a) and 8(b). Note that xe2x80x2xe2x80x3xe2x80x3 indicates differentiation with respect to z, i.e., the gradient of the trajectory. In either type, the x trajectory gives xxcex1=0 and xxcex3xe2x80x2=0 on the plane of symmetry under the same conditions for both beams.
If the initial conditions are selected in this way for the type A, then trajectory xxcex3 is focused three times and trajectory yxcex4 is also focused three times, as shown in FIGS. 7(a) and 7(b). The focused points are indicated by the arrows, respectively, in the figure. For type B, trajectory xxcex3 is focused three times but trajectory yxcex4 is focused only twice, as shown in FIGS. 8(a) and 8(b). That is, the image is turned over. The presence of these two types of OMEGA energy filters has been known for many years.
In the case of an electron microscope, an energy filter is used: (1) as a monochrometer for limiting the energy from an electron source to create a highly monochromatic beam, (2) as an electron energy loss spectroscope (EELS) for measuring the energy loss created by a specimen, and (3) as an energy filtering transmission electron microscope (EFTEM) for creating an image from zero-loss electrons excluding energy-loss electrons or from only loss electrons. The characteristics that are currently sought in these applications are to have great dispersion.
The most known method for creating great dispersion is to provide retarding. Usually, an energy filter has a dispersion of about {fraction (1/1000)} to {fraction (1/10000)} of the energies of incident electrons. Therefore, if the energies of incident electrons are lowered, the resolution of the observable energy becomes higher. For this purpose, however, the energy filter must be placed in a field across which a high voltage is applied. Consequently, this complicates the instrumentation or makes it bulky.
The present invention is intended to solve the foregoing problems. It is an object of the present invention to provide a magnetic energy filter which has an elongated beam path, has an increased sum of the absolute values of beam deflection angles, and is compact.
This object is achieved in accordance with the teachings of the present invention by a magnetic energy filter having plural magnetic fields and designed to deflect the trajectory of an electron beam from an entrance window to an exit slit, the energy filter having the following features. The filter has at least four magnetic fields. A rotational symmetry axis is located midway between the second and third magnetic fields. The magnetic fields on the opposite sides of the boundary between the second and third magnetic fields are opposite in polarity. Deflecting magnetic fields are mounted on opposite sides of a straight axis. The sum of the absolute values of the beam deflection angles in the magnetic fields, respectively, is in excess of 540xc2x0 or is about 720xc2x0.
Other objects and features of the invention will appear in the course of the description thereof, which follows.